Catalytic cracking of petroleum fractions is a well-established refinery process. The catalytic cracking apparatus per se comprises a reactor section that contains a reaction zone where fresh feed is mixed with hot regenerated catalyst under cracking conditions to form cracked products and deactivated, coked catalyst; and a regenerator section that contains a regeneration zone where the coked catalyst, after separation from volatile hydrocarbons, is burned by contact with air to form regenerated catalyst. Moving catalyst bed and fluidized bed versions of this process are used. Regardless of the design of the catalytic cracking apparatus, all present-day plants operate with a catalyst inventory that continuously circulates between the reactor section and the regenerator section. The two sections are connected by conduits through which circulation is maintained.
The cracking reaction is endothermic, requiring input of heat to maintain reaction temperature. Only a minor portion of the heat of reaction can be supplied by preheat of the hydrocarbon charge since thermal cracking and production of low octane gasoline components sets in well below the temperature maintained in modern catalytic cracking units, say 850.degree.-1000.degree. F. The necessary heat to bring the charge stock up to catalytic cracking temperature and to supply the endothermic heat of reaction is derived from the catalyst returned from the regenerator, now containing sensible heat absorbed from the heat of burning regeneration in the regenerator section. It is common practice in fluid units of modern design to control the unit for a constant preset top temperature at the point of separating spent catalyst from product vapors in the reactor. A temperature sensor at that point operates a slide valve on the conduit for return of hot regenerated catalyst from the regenerator to reactor, constraining the rate of hot catalyst return to that level which will sustain the preset top temperature in the reactor. Such "heat balanced" units respond rapidly and effectively to changes in the various operating parameters such as nature and preheat of the charge stock, regenerator temperature, catalyst activity including level of coke on regenerated catalyst, and the like.
The behavior of the regenerator, and hence the temperature and residual coke level on regenerated catalyst returned to the reactor, will fluctuate in any given unit with regenerator temperature, rate and temperature of regeneration air admitted to the regenerator. Temperature in the regenerator may be varied within limits independently of regeneration air temperature. A side stream of catalyst may be cycled through a cooler and back to the regenerator, water or steam may be introduced, usually above the fluidized bed in the regenerator to cool all or part of the regenerator. Heating effects, when needed, may be accomplished by burning a torch oil in the regenerator. More recently, additional heat input to the regenerator has been achieved by promoting combustion of carbon monoxide in the regenerator under conditions to transfer the generated heat to the catalyst.
For many years, burning of carbon monoxide in the regenerator was considered a disadvantage because that combustion took place in the "dilute phase" above the fluidized bed. The very low concentration of catalyst in the dilute phase results in the absorption by gases of substantially all the heat of oxidizing carbon monoxide to carbon dioxide, with resultant rapid rise in temperature, often to levels causing damage to internals (cyclone separators, plenum chamber and conduits) at the top of the regenerator. A common expedient to combat the effects of such "after-burning" has been to inject steam or water to areas of possible damage.
It is common practice to operate the regenerator with a limited amount of air feed so that the gaseous combustion products contain less than about 0.2 volume percent oxygen. Under such conditions, substantial concentrations of carbon monoxide (CO) are contained in the flue gas exiting from the regenerator. The actual concentration of carbon monoxide in the flue gas may vary depending on the particular plant, the nature of the catalyst and the detailed operation of the regenerator, but usually it remains in the range of about 4 to about 9 volume percent. The volume ratio of carbon dioxide to carbon monoxide (i.e. CO.sub.2 /CO ratio) normally varies from about 0.7 to about 3, and is a measure of the completeness of combustion of the reacted carbon in the coke. Thus, in operating with a limited amount of air, only about three-fourths of the total potential heat of combustion of coke is released in the regenerator itself.
Many refineries continuously feed the flue gas to a CO-boiler to complete the conversion of CO to CO.sub.2, and thus generate substantial quantities of process steam for use in the cracking process or elsewhere in the refinery. In general, the CO-boilers used differ in design from refinery to refinery, but they are generally utility boilers of the tube type. In operation, the flue gas is enriched with air and burned in the furnace of the boiler. The boiler ordinarily is equipped to accept at least one other fuel, which is used in start-up, or to supplement the fuel valve of the flue gas, or to provide process steam when the catalytic cracking apparatus itself is shut down.
The more recent developments have involved supply to the regenerator of sufficient air to convert carbon content of the coked catalyst largely to carbon dioxide and to cause oxidation of carbon monoxide to take place in the presence of catalyst at high concentration such that the heat of combustion is transferred to catalyst for use in the process by supply of heat to the reactor. One such approach is to permit temperature to rise in the dilute phase and supply catalyst thereto in amounts adequate to absorb the heat and thus protect regenerator internals while putting the generated heat to work for useful purpose. See Horecky U.S. Pat. No. 3,909,392 dated Sept. 30, 1975. A second technique is to cause the combustion of carbon monoxide to take place in the zone of high catalyst concentration, namely in the dense fluidized bed, by provision of a metal oxidation catalyst.
It has been known for some time that cracking catalysts may be modified by the addition of metal combustion promoters to increase the CO.sub.2 /CO ratio, and thus the combustion efficiency in the regenerator. The use of chromium as a promoter for moving-bed type catalytic cracking catalysts is one such example, more fully described in U.S. Pat. No. 2,647,860. In fact, a number of other metals, including nickel, deposited from the feedstock to the cracking process, are also believed to effect some degree of change in the combustion efficiency. Up until recently, however, most of the known metals had the serious drawback that, when included in the cracking catalyst in sufficient quantity to substantially affect the combustion efficiency, they also had a substantial detrimental effect on the cracking selectivity. It is well recognized, for example, that more than extremely small trace amounts of nickel in the feedstock to the cracking unit cause excessive production of coke and dry gas.
It has recently been discovered that very substantial effect on the combustion efficiency can be achieved, with little or no effect in the cracking operation, if certain Group VIII metals, more fully described hereinafter, are added to the cracking catalyst. In fact, the operation of the regenerator can be changed from partial combustion of carbon to substantially complete combustion if the cracking catalyst is promoted with as little as 2 ppm or less of platinum, for example. This development is more fully described in copending U.S. application Ser. No. 649,261, filed Jan. 15, 1976 now U.S. Pat. No. 4,072,600, the entire contents of which are incorporated herein by reference.
The platinum group metals and rhenium have high catalytic activity for oxidation of carbon monoxide and for dehydrogenation of hydrocarbons. Strangely, the oxidation activity is still effective at such low concentration that dehydrogenation activity to produce coke and hydrogen is negligible in the sense that its effect on commercial operation of a cracking unit is not detectable. These promoter metals are introduced to a cracking system by impregnating a cracking catalyst with a suitable amount of metal by impregnation with solutions of such agents as chlorplatinic acid to provide 5 ppm or 1 ppm or other suitable level of metal based on total weight of catalyst. The usually practiced method is to so impregnate the catalyst at the time of manufacture. Alternatively the metal may be added to catalyst circulating in a cracking unit by dissolving an oil soluble metal salt in the charge stock or by injecting an aqueous solution of the metal to a stream of the catalyst.
When impregnated on the catalyst, say at levels of 5 ppm or less, the whole bulk of promoted catalyst has the metal distributed as uniformly as possible through the mass. Catalyst so promoted is then used as "make-up" to an operating unit. That is, a suitable amount of such fresh catalyst is added to the circulating inventory on a continuous or intermittent basis to replace catalyst lost by attrition or deliberately withdrawn to maintain a desired level of cracking activity. Over a period of use the catalyst declines in activity, both cracking activity and metal activity for oxidation of carbon monoxide. To maintain a satisfactory average activity of the total catalyst inventory, a portion of the inventory will be withdrawn continuously or intermittently if attrition is not adequate to the purpose. Replacement of catalyst so lost or deliberately withdrawn provides an inventory of average activity needed. Thus the total inventory at any given time is made up of catalyst which is essentially inactive for both cracking and carbon monoxide oxidation, freshly added catalyst of high activity and all gradations of fading activity in between these extremes. For this purpose, a refiner will have a reserve stock of promoted catalyst. This can constitute a substantial investment in expensive promoted catalyst, particularly for plants which choose to operate in the manner described by Graven and Sailor U.S. Pat. No. 4,064,037 dated Dec. 20, 1977. According to that technique, a catalytic cracker is operated at conditions to provide high levels of carbon monoxide in the flue gas during normal operations, thereby providing fuel for a carbon monoxide fired boiler to generate steam. When the CO boiler is shut down for routine inspection and maintenance or for unscheduled reasons, additions of platinum promoted catalyst and increase in air rate to the regenerator permit continued operation without discharge of excessive amounts of carbon monoxide to the atmosphere.